It is a usual requirement for many computer applications to protect data from unwanted access by an unauthorised user. Many software protection systems are known in the art to allow only selected users to access said protected data, with the use of a password or other identification methods. Communication of data on a network is protected from undesired detection by means of encryption methods. Passwords, encryption keys and other sensitive data are usually stored in memory components in the computer systems and need to be protected even more carefully from unwanted inspection. Software control and protection methods may be not enough to stop an experienced person from bypassing these protections and tampering with the computer hardware, e.g. by direct interrogation of memory components such as integrated circuit memory.
A possible protection from the above physical attacks is to provide some kind of detecting means which detects an attempted intrusion within a protected sensitive area and reacts by giving an alarm or even by destroying any sensitive information to avoid the loss of secrecy.
U.S. Pat. No. 5,027,397 describes an intrusion barrier for protecting against mechanical and chemical intrusion into an electronic assembly. The barrier includes a screen material surrounding the electronic assembly. The screen material has formed thereon fine conductive lines in close proximity to each other in a pattern that limits the mechanical access which could be achieved without disturbing the resistive characteristic of at least one line or line segment. The lines are formed of conductive particles of material dispersed in a solidified matrix of material which loses its mechanical integrity when removed from the screen substrate. Electrical supply and signal detection means are provided which are adapted to supply a signal to the conductive lines and generate an output signal responsive to a given change in the resistance of the conductive lines whereby, when the resistance of the conductive lines changes, either as result of chemical or mechanical attack, a signal is generated which causes an alarm and the erasure of sensitive information in the protected memory component.
In order to better protect the content of the security enclosure from the most sophisticated intrusion techniques, the wires should also be invisible and not detectable. For this reason it is known to make these wires with non-metallic, x-ray transparent, (low) conductive materials, merged into a resin having color, physical and mechanical characteristics very similar to the conductive tracks. This requirement constitutes a significant constraint in the choice of the material for the resin which often provides poor electrical insulation. In some circumstances the insulation deteriorates with the increase of the temperature and this makes the detecting circuit unstable and prone to false tamper detection. This problem is due to the current leakage through the resin.
It is an object of the present invention to alleviate the above drawbacks of the prior art.